Pseudomorphic high electron mobility transistors (pHEMTs) are extensively used in wireless communication systems for switching, power and low noise amplifier applications. A pHEMT device is a semiconductor device, and illustratively includes multiple layers of gallium arsenide (GaAs) based compounds. For example, a typical pHEMT power device includes a GaAs substrate, an epitaxial GaAs buffer layer, an AlGaAs/GaAs superlattice layer, an undoped AlGaAs layer, may include a first doped silicon layer disposed over the superlattice, will include a first spacer, an InGaAs channel layer, a second spacer, an upper doped silicon layer which will typically have a different doping concentration from the first doped silicon layer, an undoped AlGaAs layer, an undoped GaAs layer, and a highly doped GaAs cap layer stacked sequentially on a semi-insulating GaAs substrate. The device includes a source electrode and a drain electrode formed in ohmic contact with the highly doped GaAs cap layer; and a Schottky metal gate electrode formed on the undoped AlGaAs layer.
Power amplifiers that are constructed in known pHEMT technologies often suffer drain and gate lag, and other non-idealities such as kink effects. These non-idealities are in part due to the presence of deep level traps in the substrate and at the interface of the substrate and buffer layer. These traps may change charge state during some modes of device operation. These states have a comparatively long lifetime and introduce slow device current transients which can adversely impact the application under some conditions. For example, non-ideal kink characteristics contribute to difficulties constructing some amplifier bias circuits. More significantly, as a result of the relatively slow drain lag, the power amplifier cannot adequately track rapid changes in the applied drain voltage (Vd) on the final stage of the amplifier. This adversely impacts the utility of known pHEMT devices for some modern amplifier power conservation schemes, for example those obtained by dynamic changes to the amplifier drain voltage. As such, known power amplifiers having known pHEMTs are unacceptable in many applications.
What is needed, therefore, is a pHEMT that overcomes at least the referenced deficiencies.